One.relative_identifier(
  1 \\ 2,
  key: 3 \\ 4
) \\ Two.relative_identifier(
  2 \\ 1,
  key: 4 \\ 3
)
One.relative_identifier(
  1 \\ 2,
  key: 3 \\ 4
)(
  4 \\ 5,
  key: 6 \\ 7
) <- Two.relative_identifier(
  2 \\ 1,
  key: 4 \\ 3
)(
  5 \\ 4,
  key: 7 \\ 6
)
One.relative_identifier key: 1 \\ 2
One.relative_identifier unqualified 1 \\ 2,
                                    key: 3 \\ 4
One.relative_identifier 1 \\ 2,
                        key: 3 \\ 4